Schedule I - Parent Company Financial Information (Condensed Statements of Income) (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues:
Total revenues	$ 13,361	$ 13,363	$ 13,732	$ 13,833	$ 12,328	$ 12,238	$ 12,222	$ 11,712	$ 54,289	$ 48,500	$ 41,313
Expenses:
Operating costs									7,318	7,639	6,355
Depreciation									354	328	309
Interest									186	192	140
Gain on sale of business									270	0	0
Benefit for income taxes									1,155	1,023	690
Net income	$ 101	$ 314	$ 431	$ 430	$ 145	$ 290	$ 344	$ 368	1,276	1,147	1,231
Parent Company
Revenues:
Management fees charged to operating subsidiaries									1,469	1,509	1,370
Investment and other income, net									5	4	3
Total revenues									1,474	1,513	1,373
Expenses:
Operating costs									1,370	1,434	1,366
Depreciation									252	212	193
Interest									186	192	141
Total operating expense									1,808	1,838	1,700
Loss before gain on sale of business, income taxes and equity in net earnings of subsidiaries									(334)	(325)	(327)
Gain on sale of business									270	0	0
Loss before income taxes and equity in net earnings of subsidiaries									(64)	(325)	(327)
Benefit for income taxes									(70)	(81)	(107)
Loss before equity in net earnings of subsidiaries									6	(244)	(220)
Equity in net earnings of subsidiaries									1,270	1,391	1,451
Net income									$ 1,276	$ 1,147	$ 1,231